1. Field of the Invention
This invention relates to an imagery lens system which is used in an optical system for an image reading system such as a facsimile, an image scanner or the like, and more particularly to an imagery lens system for magnifying or reducing an image to be read.
2. Description of the Prior Art
An imagery lens system which is used in an optical system for an image reading system such as a facsimile, an image scanner or the like in order to focus an image of a manuscript on an image pickup device such as a CCD on a magnified or reduced scale is required to be high in resolution, large in marginal light quantity, and small in distortion at a magnification used.
As the lens systems which satisfy such requirements, there have been provided lens systems of Gauss type, Ortho metar type and Xenotar type. See U.S. Pat. Nos. 4,264,139 and 4,415,241, Japanese Unexamined Patent Publication No. 59(1984)-90810, and the like.
The known lens systems comprises five or more lens elements, and accordingly large in size and high in cost. In order to reduce the size and the cost of the lens system, thereby reducing the size and the cost of the overall optical system, lens systems of Tesaar type comprising four lens elements have been developed as disclosed, for instance, in Japanese Unexamined Patent Publication No. 1(1989)-183614, and U.S. Pat. No. 4,781,449.
However, since the Tesaar type lens system for an image reading system having four lens elements is apt to be large in the overall length, there has been a demand for a lens system for an image reading system which is short and small in the overall size and can be manufactured at low cost.